The present check valve, while useful for other applications, is particularly adapted for use on a fire engine pumper having a water reservoir in which the reservoir is filled from a water source such as a fire hydrant. The check valve is urged to a closed position by a spring thereby insuring that the inlet fluid pressure must be sufficient to overcome any hydrostatic pressure in the fire engine reservoir before the valve will open. The improved check valve of the present invention also includes an enlarged body cavity which insures that the flow passages through the valve, when the valve is opened, is sufficient to prevent any restriction in an area of the valve as the fluid flows therethrough. Furthermore, locking means are provided to lock the check valve in a closed position so that when the fire engine pumper dispenses water from the reservoir any vacuum pull on the reservoir will not cause the check valve to open.